This invention relates to a method of detecting abnormality in various systems or devices of an internal combustion engine, such as a catalyst system, evaporative fuel-purging system, fuel supply system, air-fuel ratio sensor and ignition system.
A method of detecting deterioration of a three-way catalyst in an internal combustion engine having such a catalyst in the exhaust pipe, and an upstream air-fuel ratio sensor and a downstream air-fuel ratio sensor arranged in the exhaust pipe upstream and downstream of the catalyst to detect the air-fuel ratio of exhaust gases from the engine, has already been proposed by the assignee of the present application (Japanese Patent Application No. 2-117890).
According to this proposed method, deterioration of the three-way catalyst is detected by comparing a first area between a first signal and a third signal in the signal waveform output by the upstream air-fuel ratio sensor with a second area between a second signal and a fourth signal in the signal waveform output by the downstream air-fuel ratio sensor, based on the first and second signals output respectively by the upstream and downstream air-fuel ratio sensors when the fuel supply to the engine has been increased, and based on the third and fourth signals output respectively by the upstream and downstream air-fuel ratio sensors when the fuel supply to the engine has been stopped; and by comparing the temperature of the catalyst with a first temperature exhibited by the catalyst when the HC purifying percentage .eta. is 50%, and with a second temperature exhibited by the catalyst depending on the engine load.
A method of detecting deterioration of an exhaust gas component concentration sensor in an internal combustion engine, wherein the amount of fuel supplied to the engine is feedback-controlled based on a correction value for correcting the air-fuel ratio, determined according to an output signal from this sensor, has also been proposed by the assignee of the present application (Japanese Patent Application No. 2-142825).
According to this proposed method of detecting deterioration of an exhaust gas component concentration sensor, the deviation between the output signal from the sensor and a predetermined reference value outside the output range of the sensor that can be assumed when it is functioning normally is integrated, and the deterioration of the sensor is assessed by comparing the integral value with a predetermined deterioration reference value.
If however the amount of fuel remaining in the fuel tank decreases, problems arise with the above conventional method as follows.
In the aforementioned method of detecting deterioration of a three-way catalyst, the catalyst is judged to have deteriorated if the difference between the first and second areas is smaller than a predetermined value, and at the same time if the temperature of the three-way catalyst is lower than the second temperature. However, if the amount of fuel remaining decreases, there is a possibility of the fuel pump sucking in air in which case the air-fuel mixture supplied to the engine will be leaner.
In general, when the air-fuel mixture becomes learner, the catalyst temperature rises. If the amount of fuel remaining decreases, therefore, the catalyst temperature will rise to exceed the second temperature, and there was thus a risk that the three-way catalyst could be judged to be functioning normally when in fact it had deteriorated.
Further, in the method of detecting deterioration of an exhaust gas component concentration sensor, if the amount of fuel remaining decreases and the gas mixture becomes leaner, the output signal from the sensor tends towards the lean side, so that the accuracy of detecting deterioration by comparing the integral value with the predetermined deterioration reference value, declines.
In addition, there are other problems in the technology of the prior art apart from those described above.
When air is sucked in from the fuel pump due to a decline in the amount of fuel remaining, insufficient fuel is supplied to the engine and the engine speed temporarily falls. In a method to detect abnormality of the ignition system wherein ignition failure in the cylinder is detected by monitoring a change in engine speed, therefore, there was a risk that ignition failure might be judged to have occurred when in fact it had not.
Similarly, in a method to detect abnormality of the fuel supply system by monitoring a change in the value of an air-fuel ratio correction coefficient set value of an air-fuel ratio correction coefficient set according to an output signal from an exhaust gas component concentration sensor, abnormality could not be detected accurately due to a temporary insufficiency in the fuel supply caused by a decrease in the amount of fuel remaining in the fuel tank.
Further, in a method to detect abnormality in an evaporative fuel-purging system by monitoring the amount of evaporative fuel purged into the intake system from a change in the value of an air-fuel ratio correction coefficient, abnormalities could not be detected accurately as the amount of evaporative fuel generated in the fuel tank decreased due to a decrease in the amount of fuel remaining therein.